The JOY Mall is where you can spend your JOY earnings. You can spend it on gold, characters, skill cards, a limited time skill card or character, or VIP items.
The limited time skill card is priced at 30B / 50B, and gives a specific skill card of random rarity between ★3 to ★5. The limited time character is priced at 20B, and gives a specific character of random rarity between ★3 to ★5. However, if it is a Premium character, the rarity will be of between ★5 to ★5 Diamond and cost 90B.
Purchasing in bulk is generally better, with the strange exception of purchasing 3 Skill Card tickets being more expensive per ticket.